Vintage bike modification

[Ajax Bay]

A six speed block will fit just fine on your current wheel. Don't waste time with more (ie than 6). A 14-28 freewheel is easy to obtain and the RDs of the day will work (I have a Campagnolo Nuovo Record Pat 76 which works with a 14-28 without hiccup).

You can get big sprockets on a 5 speed block

You can but I bet the current RD would not operate with it (ie the one imaged).
The bottom bracket will be square taper. I would not fit a triple but find a compact (ie double) which fits the bottom bracket. Your FD, which will be friction, will likely work. And you then have a 34/28 (ie 33" length gear) which ought to get you up any hills you choose to address (YPMV). And a 50/14 (ie 96" length gear) which will take you up to a speed beyond which it's not really worth pedalling, certainly when out on such a bike.

Old bicycles are meant to be ridden so go ahead and convert . . . Assuming you have friction shifters they will probably cope with an extra gear or two - but your derailleurs might not . . . . . You may be able to drop to a smaller double up front (possibly just a chain ring) and a wider range rear block - if it will achieve a low enough gear for you. So at best: swap front small chain ring (avoiding new bottom bracket and possibly new front derailleur)

Your crankset will probably not take a smaller chainring (well maybe a couple of teeth smaller but that's it) because of the BCD bitd.

Since you ask, I think that you are in danger of 'spoiling the bike' (OP's words) if you go further than this (a triple would be fine, but at considerable cost and faff). But I also agree with @mrandmrspoves , "Old bicycles are meant to be ridden".

 

[raleighnut]

A six speed block will fit just fine on your current wheel. Don't waste time with more (ie than 6). A 14-28 freewheel is easy to obtain and the RDs of the day will work (I have a Campagnolo Nuovo Record Pat 76 which works with a 14-28 without hiccup).

You can but I bet the current RD would not operate with it (ie the one imaged).
The bottom bracket will be square taper. I would not fit a triple but find a compact (ie double) which fits the bottom bracket. Your FD, which will be friction, will likely work. And you then have a 34/28 (ie 33" length gear) which ought to get you up any hills you choose to address (YPMV). And a 50/14 (ie 96" length gear) which will take you up to a speed beyond which it's not really worth pedalling, certainly when out on such a bike.

Your crankset will probably not take a smaller chainring (well maybe a couple of teeth smaller but that's it) because of the BCD bitd.

Since you ask, I think that you are in danger of 'spoiling the bike' (OP's words) if you go further than this (a triple would be fine, but at considerable cost and faff). But I also agree with @mrandmrspoves , "Old bicycles are meant to be ridden".

Square taper crank on a 72 frame, possible but I doubt it, however changing a cottered shaft to a square taper one is no biggie (I'd avoid fitting a 'cartridge' BB)

 

[TheDoctor]

If I can chuck in my two pennorth...
My Carlton has a 50-30 double chainset, which strikes me as somewhat compact. Thing is, that's what it came with in 1970-something.
A wide ratio block and compact square taper chainset would be in keeping IMHO.

 

[Ajax Bay]

Square taper crank on a 72 frame, possible but I doubt it,

Of course this may be the case, but I think you underestimate the likelihood it is a square taper: see these quotes:
"The first successful aluminum cranks were introduced in 1933 by Stronglight. They used a square taper fitting on the spindle instead of the cotters used by most steel cranks at the time. As their name implied, these cranks were strong and light."
"the square taper design was gradually adopted during the 1960s by almost all chainset manufacturers and of course became the standard until Shimano broke the mould with their splined chainset fittings on MTB chainsets in the middle 1990s which later filtered down to the road sets."

My [1970-something] Carlton has a 50-30 double chainset,

What type/model of chainset, please? Getting a 30t on that age chainset is unusual, I think.

 

[Poacher]

@taximan, what size chainrings do you currently have on your Hugh Porter? Reason for asking is that I snapped up a bargain Holdsworth a few years ago which was bloody hard work up hills due to the 52/42 chainset (144mm BCD, so 42 was as low as it would go!). Simply swapping to a 50/34 semi-compact chainset made the world of difference; everything else, including the block and derailleurs, stayed as they were.

 

[raleighnut]

Of course this may be the case, but I think you underestimate the likelihood it is a square taper: see these quotes:
"The first successful aluminum cranks were introduced in 1933 by Stronglight (photo at the top). They used a square taper fitting on the spindle instead of the cotters used by most steel cranks at the time. As their name implied, these cranks were strong and light."
"the square taper design was gradually adopted during the 1960s by almost all chainset manufacturers and of course became the standard until Shimano broke the mould with their splined chainset fittings on MTB chainsets in the middle 1990s which later filtered down to the road sets."

What type/model of chainset, please? Getting a 30t on that age chainset is unusual, I think.

The first time I saw a square taper crank on a mates bike was in about 76 and we all wondered how you would be able to get it off in order to clean/replace the bearings (Cottered cranks only required a couple of spanners and a big hammer to strip em down)

 

[TheDoctor]

@Ajax Bay I'll have a look tomorrow. I doubt it's anything all that exotic, as the bike also has Simplex retrofriction levers and a Duopar Eco rear mech.
Frankly, I have little use for a 50T ring. My main bike has a 12-32T cassette and a 36T chainset. I'm a bit slowspinny.
Here's the thread just after I got it. It's an SR chainset, apparently.

 

[taximan]

@taximan, what size chainrings do you currently have on your Hugh Porter? Reason for asking is that I snapped up a bargain Holdsworth a few years ago which was bloody hard work up hills due to the 52/42 chainset (144mm BCD, so 42 was as low as it would go!). Simply swapping to a 50/34 semi-compact chainset made the world of difference; everything else, including the block and derailleurs, stayed as they were.

Hi Poacher
The bike is fitted with 52/42 chainrings at the moment and to be honest, I struggle on the hills now, hence the desire for a lower gear.

Mike

 

[taximan]

Thank for all your advice guys, The bike has a square taper & cartridge BB which I imagine was fitted by one of its previous owners but I think everything else is original. I really enjoy riding Hughy but sadly I can no longer give him the thrashing he deserves but lower gearing would at least keep us on the road a bit longer

 

[Ajax Bay]

The bike has a square taper & cartridge BB

Your challenge will be finding a chainset that has a small enough BCD for the inner, in a style old enough to pass muster against the 'not spoiling it' criterion, or accepting that you'll have to accept a degree of compromise. My crankset (on my older bike) is an SR Apex but 40t is the smallest I could get/have found.
Researching, looking for and finding what you need can/could be fun. Maybe Retrobike might be an avenue?
Thanks for the Esk Valley info.

 

[Poacher]

It causes me a considerable degree of sorrow that Stronglight show no sign of reintroducing their old 86mm BCD standard, which allows an inner down to 28t while still being stiff enough to fit an outer of 54t or even more (not that I'd suggest quite such a wide gap on a double chainset). I have a 46/30 on my refurbished Sid Standard Superbe, which works well with an 11-28 8 speed cassette, and have a small stock of new rings for replacement when necessary. Lucky me!
If you've decided on a triple, I'd suggest having a look at Spa's range (they don't pay me, honestly) - they currently have their own XD-2 triple 48/38/28 with a choice of crank lengths from 160mm to 175mm for a bargain £35. Might even be worth popping over to Harrogate to introduce yourself and check over their stock. Can't be far from your home in Whitby, after all, they're in the same county.

 

[taximan]

It causes me a considerable degree of sorrow that Stronglight show no sign of reintroducing their old 86mm BCD standard, which allows an inner down to 28t while still being stiff enough to fit an outer of 54t or even more (not that I'd suggest quite such a wide gap on a double chainset). I have a 46/30 on my refurbished Sid Standard Superbe, which works well with an 11-28 8 speed cassette, and have a small stock of new rings for replacement when necessary. Lucky me!
If you've decided on a triple, I'd suggest having a look at Spa's range (they don't pay me, honestly) - they currently have their own XD-2 triple 48/38/28 with a choice of crank lengths from 160mm to 175mm for a bargain £35. Might even be worth popping over to Harrogate to introduce yourself and check over their stock. Can't be far from your home in Whitby, after all, they're in the same county.

Not far at all, only 65 miles or so

 

[taximan]

I called round my local bike shop today and he is going to do the work for me. We will be fitting modern parts and as suggested on here I will be keeping anything original that is replaced. Thanks for all your help and suggestions Guys